Production of rubber products



Patented Aug. 28, 1951 Lynn Harbison, Akron, Ohio, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application October 18, 1948,

Serial No. 55,242

15 Claims. (01. 260-780l This invention relates to the production of rubber products. In one of its more specific aspects it relates to the retardation of vulcanization of rubber mixes. In still another one of its more specific aspects it relates to retarding too rapid acceleration of vulcanization caused by high pH carbon black. In still another of its more specific aspects it relates to new vulcanized rubber products.

In the manufacture of molded rubber products, crude rubber is milled with the desired fillers, softeners, sulfur and accelerators. In the first stage of plant processing, carbon black and other ingredients, except the accelerator of vulcanization and sulfur, are added with the mixing temperature reaching a maximum in the range of 290 to 330 F., especially for tread compounds.

A mixed stock, whether mill mixed or Banbury' mixed, is cooled with'a water spray in order to aid in reducing the temperature of the mixed stock to room temperature as soon as possible. After this first stage mixing, the rubber mix is placed on what is termed a sheet-out mill and the mixed stock is cut in slabs and subjected to a water spray. These slabs are then cooled in the air, usually for about one-half to three-quarters of an hour, and then placed on a skid to age, generally for a minimum of 5 hours. The second step is to take this master batch form and add the accelerator and sulfur in a Banbury mixer or on a' mill. During this step the temperature should not be greater-than 230 'F., decidedly cooler than the original mix when the carbon black is added. This final mix is cooled with a water spray, air-cooled and then aged' at room temperature. This mixed stock is cooled as rapidly as possible in order to avoid scorched material. endering or tubing ,which is usually carried on in the range of 150 to 300 F. 'The calendered or tubed product is then vulcanized The most common'vulcanizing temperature for tires is 260 to 300 F. For very large tires the curing temperature may be dropped to as low as 260 F. In the case of molded industrial products,'a temperature of about 320',F., is the upper range at which vulcanization is carried out. In carrying out the plant processes of second stage mixing and of calendering or tubing, it is necessary that the mixed rubber is not partially vulcanized pre- The next step in processing is cal-' maturely. If vulcanization occurs prematurely in factory processing, factory operations cannot be carried out efliciently. This premature vulcanizationis known as scorching.

In recent years many new carbon blacks have I become available to the rubber industry. Chief among these new carbon blacks are combustion furnace carbon blacks, which 'in general havea higher pH than the older type of channel black or thermal furnace carbon blacks. Combustion furnace carbon blacks have a pH of from 8 to 10.5, usually from 8.6 to 10.1. For instance, one commercially available high abr'asion combustion furnace (HAF) black, has a pH of approximately 9.1 and a commercially available high modulus combustion furnace (HMF) black, hasa pH of approximately 9.7. In contrast to the pH of these combustion furnace blacks, channel blacks usually have a pI-I value on the acid side inthe neighborhood of 3.8 to 6."

Furnace blacks such as those just mentioned have many, characteristics which add useful properties to'the finished rubber product. Thus, the HAF black' has a high degree of structure and rubber products containing this black have exceptionall high'resistance to abrasion; There are several high pH furnace blacks now on the market. which add valuable properties to the finished rubber product. These high pH carbon blacks have caused considerable difficulty to the rubber compounder and processor as pointed out by C. W. Sweitzer and W. C. Goodrich in The Rubber Age, volume 55, No. 5, August "1944, at page 471. The pH of the carbon black has a decided eiTect on the rate of vulcanization. High pH' carbon blacks accelerate cure or vulcanization. Sweitzer and Goodrich point out that due to this factor economy inorganic acceleration is permitted. However, in most cases changing the recipe of a rubber mix, that is, changing the amount of organic accelerator and sulfur, also changes the properties of the finished rubber product. It has an effect on such properties as tensile strength, heat build-up, the compression set, the hardness, the resistance to abrasion, the

modulus of elasticity, the elongation, etc. Accelerator starvation with HAF black results in low abrasion resistance. The rubber compounder usingthese high pH carbon blacks has been conironted with the problem of maintainin the desired properties in the finished product by maintaining the proper amount of organic accelerator and sulfur while trying to overcome the problem of scorch or premature vulcanization. Well-known retarders of vulcanization such as N-nitroso diphenyl amine, rosin and salicylic acid in a dispersing agent have been unsuccessful in overcoming the problem.

I have discovered that zinc chloride will retard vulcanization of a rubber mix in which mix vulcanization has been accelerated due to a high pH carbon black. Incorporating zinc chloride into a rubber mix before it is vulcanized. will eifectively' slow up vulcanization so that the rub-' ber mix may be processed without encountering scorch.

One object of this invention to provide a method of retarding vulcanization of rubber mixes. I I

Another object of this invention is to provide a method by which premature: vulcanizationcaused by the use of high pH carbon black inco n vulcanization, as

scorch during the processing of rubber products, must be prevented in order to allow the rubber mix to be preformed and shaped before it is cured or vulcanized. Testing methods, run on test samples, have been devised and standards have been set so as to determine if a; particular rubber mix will scorch during processing. The most frequently-used testing. method is that run on a Mooney shearing disc viscometer. These Mooney viscometers come equipped with a large rotor (1 inches in diameter) and a small rotor (1.1 3. inches in diameter). The viscometer can be set to operate at a; given temperature. The Mooney scorch testis made byusing' the small rotor with the machine usually set to operate at 250 F., that is, the temperature. of the rubber sample on which the scorch test is being run is held at 250 F. Primarily, the instrument con sists of a disc rotor which turns in a shallow cylindrical chamber recessed into two horizontal platens. The surfaces of the chamber and rotor are knurled to prevent slippage; The sample is formed in place around the rotor as the shamber'is closed. The. rubber isshearedby the rotor which is driven by a sychronous motor. The resistance of the rubber to this shearing action I develops a thrust in a floating horizontal worm shaft which presses against a deflecting U'-shaped steel spring- The deflection is' read on a dial gauge and is proportional to the true mean vis cosity of th sample. In determining the scorch time, deflection readings are taken at one min ute intervals until the deflection of the indicator increases appreciably. At this point the" viscosity of the rubber mix sample has increased duev to the beginning of vulcanization. The Mooney scorch time is then given as the time in minutes at which vulcanization began, indicated by the appreciabl increase in viscosity readings.

In this disclosure Mooney scorch time when used means the time in minutes at which the viscosity of the sample began to increase appreciably because of vulcanization. Mooney shearing disc viscometers are widely used in the rubber industry, and one skilled in the art will have no difficulty in determining and understanding what I mean by the Mooney scorch time.

I have discovered that zinc chloride will effectively retard vulcanization of rubber mixes which vulcaninze too rapidly due to the use of a high pH carbon black. The zinc chloride can be incorporated in the rubber mix before, during or after the other ingredients are added. In practicing my invention, the zinc chloride can be added either on a mill or in a Banbury. mixer or it may be mixed with the carbon black before the black is added in the first stage of mixing. Zinc chloride is a hygroscopic compound and for this reason can be more readily handled in the factory in master batch form with stearic or oleic acid or oilscommonly incorporated into rubber mixes. The zinc chloride used in the practice of my invention is useful. and will retard too rapid vulcanization caused by a high pH carbon black in rubber mixes containing natural, synthetic or reclaimed rubber, or various mixtures of the above-mentioned types of rubber. The zinc chloride of my invention is particularly adaptable to the problems of the tread manufacture in which the rubber mix from which the tread is formedcontains from 40 to 65 parts of carbon black per parts of rubber, usually natural rubber or the new synthetic copolymers butadiene-styrene pro-' duced at low temperatures, such as GRS rubber and recently developed low-temperature variants thereof. Most rubber mixes contain from '10 to 65 parts of carbon black per IDOparts of rub her. The problem of too rapid vulcanization be comes greater as the amount ofhigh pH carbon black is increased; therefore, it is-necessary that a greater amount of zinc chloride be added as the amount of carbon black is increased in order to effectively retard vulcanization. In practicing my invention, one skilled in the art will have no trouble in determining the amount of zinc chlo-' ride necessary to effectively retard vulcanization for any particular recipe or rubber mix. The Mooney scorch time for any particular rubber mix may be determined and the addition of zinc chloride will increase this Mooney scorch time so that there will be no problem of premature ing vulcanization in rubber. mixes containing benzothiazyl disulfide or its derivatives such as zinc benzothiazyl sulfide and acyl benzothiazyl sulfide. My invention; is also particularly adaptable to those rubber mixescontaining mercapto= benzothiazole and its derivatives such as the dinitro phenyl ester or mercaptobenzothiazole; Other widely used accelerators of vulcanization such as thiuram monosulfide or" its derivatives also give too rapid vulcanization when used-with high pH carbon blacks, and zinc chloride will effectively retard vulcanization when these ac' celerators are used along with. high pH- carbon blacks. Sometimes a rubber compounderwill use more than one accelerator in a given rubber mix. Zinc chloride will effectively retard vulcanization when a mixture of organic. accelerators is used in-conjunction with a high pH carbon black. For instance, rubber mixes in which a benzothiazyl disulfide or a mercaptobenzothiazole is the. accelerator usually contain from 0.6 to 2.5 parts of the accelerator per 100 parts of rubber and 2 to 3 parts of sulfur per. 100 parts of rubber. Zinc chloride in the range of 0.5 to 3.5 parts per 100 parts of rubberwill effectively retard vulcanization or increase the Mooney scorch time of such a rubber mix. 0.8 to 1 part of a benzothiazyl disulfide orv a mercaptobenzothiazole per 100 parts of rubber with 2.00 to 2.75 parts of sulfur per 100 parts of rubber will cover the most commonly practiced range. I have found'that 1 to 2 parts of zinc chloride per 100 parts of rubber will effectively retard vulcanization or increase the Mooney, scorch time of such a mixture. The Mooneyscorch time, as run on test samples, gives a good indication of the process scorching possibilities of a particular rubbermix. In the following examples of my invention the amounts,.

particular ingredients and conditions are given as typical and are not to be construed so as to narrow the scope of my invention.

The antioxidant used in the following examples was Thermoflex A which is a mixture of 25% di-p-methoxy-diphenylamine, 25% diphenyl-pphenylene-diamine and 50% phenyl-B-naphthylamine.

Example I Three master batches were made up each containing the following:

7 Parts by weight Smoked sheets 100 HAF carbon black (pH 9.1) 50 Zinc oxide A Antioxidant 1 Stearic acid 3 L. V.asphaltum oil 3 Sulfur 2.5

Six-tenths of a part of benzothiazyl disulfide was added to the first master batch. A Mooney scorch test was run on a sample, giving a scorch time of 12.5 minutes at 250 F.

One part of zinc chloride was added to the was added to the second master batch. A Mooney scorch test was run on a sample, giving a Mooney scorch time of 9.5 minutes at 250 r One part of zinc chloride was added to the rubber mix containing the mercaptobenzothiazole, and a Mooney scorch test was run on a sample, giving a scorch time of 21 minutes at 250 F. This 11.5 minute increase in Mooney scorch time was sufiicient to eliminate scorching during processing.

Seven-tenths of a part of zinc ,benzothiazyl sulfide was added to the third master batch, and a Mooney scorch test was run on a sample, giving a Mooney scorch time of 14.5 minutes at 250 F.

One part of zinc chloride was added to the rubber mix containing the zinc benzothiazyl sulfide, and a Mooney scorch test was run on a.

sample, giving a Mooney scorch time of 25 minutes at 250 F. Here again the increasein scorch time was sufficient to eliminate any scorching during processing.

One-tenth of a part of thiuram monosulfide and four-tenths of a part of mercaptobenzothiazole were added to the master batch and a Mooney scorch test was run on a sample, giving a Mooney scorch time of 15 minutes at 250 F.

One part of zinc chloride was addedto the 7 rubber mix containing the thiuram monosulfide and mercaptobenzothiazole, and a Mooney scorch test Was run on a sample, giving a scorch time of 26 minutes at 250 F. This 11 minute increase in scorchtime was sufficient to eliminate any problem of scorching during processing.

As will be evident to those skilled in the art, various modifications of this invention can be made, or followed, in the light of the foregoing disclosure and discussion, without departing from the spirit or scope of the disclosure or. from the scopev of the claims.

.I claim:

1. In the process of making rubber products wherein the rubber mix, containing an organic accelerator of vulcanization, sulfur and a high pH furnace carbon black having a pH of from 8.6 to 10.5, is preformed and vulcanized, that improvement of retarding vulcanization which consists of adding zinc chloride as the retarder to said rubber mix in an amount of 0.5 to 3.5 parts per parts of rubber.

2. In the process of making rubber products wherein the ruber mix, containing from 0.6 to 2.5 parts of benzothiazyl disulfide per 100 parts of rubber, 2 to 3 parts of sulfur per 100 parts of rubber and from 10 to 65 parts of a furnace carbon black having a pH of from 8.6 to 10.1 per 100 parts of rubber, is preformed and vulcanized, that improvement of retarding vulcanization which consists of adding zinc chloride as the retarder to said rubber mix in an amount of from 0.5 to 3.5 parts per 100 parts of rubber.

I 3. In the process of making natural rubber products wherein the rubber mix, containing from 0.6 to 2.5 parts of benzothiazyl disulfide per 100 parts of natural rubber, 2 to 3 parts of sulfur per 100 parts of natural rubber and from 10 to 65 parts of a furnace carbon black having a pH of from 8.6 to 10.1 per 100 parts of natural rubber, is preformed and vulcanized, that improvement of retarding vulcanization which consists of adding zinc chloride as the retarder to said natural rubber mix in an amount of from 0.5 to

II treads wherein the rubber mix, containing from,

aseaua fur per 100 parts of natural rubber and from 407 to 65 parts of a high abrasion furnace carbon black having a pH of from 8.6 to 10.1 per 100' parts. of natural rubber, is preformed and vulcanized, that improvement of retarding vulcanization which consists of adding zinc chloride as the retarder to said rubber mix in an amount of from 1 to 2 parts per 100 parts of natural rubber.

5. In the process of making rubber products wherein the rubber mix, containing from 0.6 to 2.5 parts of mercaptobenzothiazole per 100 parts of rubber, 2 to 3 parts of sulfur per 100 parts of rubber and from to 65 parts of a furnace carbon black having a pH of from 8.6 to 10.1 per. 100 parts of rubber, is preformed and vulcanized, that improvement of retarding vulcanization which consists of adding zinc chloride as the retarder to said rubber mix in an amount of from 0.5 to 3.5 parts per 100 parts of rubber.

6. In the process of making natural rubber treads wherein the rubber mix, containing from 0.8 to 1 part of a mercaptobenzothiazole per 100' parts of natural rubber, 2 to 2.75 partsof sulfur per 100 parts of natural rubber and from 40 to 65 parts of a high abrasion furnace carbon black having a pH of from 8.6 to 10.1 per 100 parts of natural rubber, is preformed and vulcanized, that improvement of retarding vulcanization which consists of adding zinc chloride as the retarder to said rubber mix in an amount of from 1 to 2 parts per 100 parts of natural rubber.

7. In the process of'making rubber products wherein the rubber mix, containing. benzothiazyl disulfide, sulfur and a high pH furnace carbon black having a pH of from 8.6 to 10.5, is preformed and vulcanized, that improvement of retarcllng vulcanization which consists of adding zinc chloride as the retarder to said rubber mix in an amount of from 0.5 to 3.5 parts per100 parts of rubber.

8. Inthe process of making rubber products wherein the rubber mix, containing a mercaptobenzothiazole, sulfur and a high pH furnace carbon black having a pH of from 8.6 to 10.5, is preformed and vulcanized, that improvement of re-- tarding vulcanization which consists of adding. zinc chloride as the retarder to said rubber mix in.

an amount of from 0.5 to 3.5 parts per 100 parts of rubber.

9. In the process of making rubber products wherein the rubber mix, containing an organic accelerator of vulcanization, sulfur and from 10 to 65 parts of a furnace carbon black having a pH 01 from 8.6 to 10.5 per 100 parts of rubber, is

' preformed and vulcanized, that improvement of retarding vulcanization which consists of adding zinc chloride as the retarder to said rubber mix in an amount of from 0.5, to 3.5 parts per 100 parts of rubber.

10. In the process of making rubber products wherein the rubber mix, containing an organic accelerator of vulcanization, sulfur and from40 to parts of a furnace carbon black havinge. pH- of from 8.6 to 10.5per parts of rubber ial preformed and vulcanized, that improvement of.

retarding vulcanization which consists of adding? zinc chloride as the retarder to said rubber in an amount of from 1 to 2 parts per 100 parts of; rubber.

- 11. A vulcanizable rubber mix comprising, rub-- ber, an organic accelerator of vulcanization, sulfunfurnace carbon black having a pH of from 8.6 to 10.5 and a vulcanization retarder consisting; of' zinc chloride in an amount of from 0.5 to 315 parts per 100 partsof said rubber.

12. A vulcanizable rubber mix comprising. rub ber, benzothiazyl disulfide in an amount of from:

0.6 to 2.5 partssulfur in an amount of from 2 1:03 parts, furnace carbon black having a pH of from- 8.6 to 10.5 'in an amount of from 10 to 65 parts and a vulcanization retarder consisting. of. zinc; to 3.5 parts, 311

chloride in an amount of from 05 partsper 100 parts of said rubber; w 13. The: composition of matter of claim 12:

wherein saidbenzothiazyl disulfide is present in an amount of from .8 to 1 part, said sulfur is present in an amount of from 2 to 2.75 parts; said furnace carbon black is present in an amountof from 40 to 65 parts and said zinc chloride present in an amount of from 1 to 2 parts. all parts per 100 parts of said rubbe 14. A vulcanizable rubber mix comprising, rubber, mercaptobenzothiazole in an amount offromi 0.6 to 2.5 parts, sulfur in an amount of from 2 to 3' parts, furnace carbon black having a pH- of'irom 8.6 to 10.5 in an amount of from 10 to- 65 parts and a vulcanization retarderconsisting of zinc chloride in an amount of from 0.5 to 3.5 parts, all parts per 100 parts of said'rubber'. 15. The composition of matter of claim- 14 wherein said mercaptobenzothiazole is present in an amount of from 0:8 to 1 part, said sulfur-is present in an amount of from 2 to 2.75 parts, said furnace carbon black is present in an amount of from 40to65partsandsaid zinc chloride is present in anamount of from 1 to 2 parts, all parts per 100-parts of said rubber.

' LYNN HARBISON-L REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Can Chem. 8; Process Inds." of March 1944, pages 151-162. v I India Rubber World, vol. 119, October 1948..

pages 57-62. 

9. IN THE PROCESS OF MAKING RUBBER PRODUCTS WHEREIN THE RUBBER MIX, CONTAINING AN ORGANIC ACCELERATOR OF VULCANIZATION, SULFUR AND FROM 10 TO 65 PARTS OF A FURNACE CARBON BLACK HAVING A PH OF FROM 8.6 TO 10.5 PER 100 PARTS OF RUBBER, IS PREFORMED AND VULCANIZED, THAT IMPROVEMENT OF RETARDING VULCANIZATION WHICH CONSISTS OF ADDING ZINC CHLORIDE AS THE RETARDER TO SAID RUBBER MIX IN AN AMOUNT OF FROM 0.5 TO 3.5 PARTS PER 100 PARTS OF RUBBER. 